The conventional method for drilling bore holes in the earth to recover hydrocarbons, either oil or gas or a mixture of both, entails drilling a relatively large diameter surface bore for a few hundred feet and cementing surface casing in the bore hole to provide a seal with the surface. A stack of valves referred to as the blow out prevention (B.O.P.) stack is then connected to the top of the surface casing. Drilling operations are then carried out through the B.O.P. stack. A drill bit is attached to the lower end of heavy drill collars which are supported by joints of drill pipe, all of which are threadedly interconnected. The drilling rig includes a derrick with appropriate hoist means for assembling the drill string joint-by-joint in a vertical stack and lowering the string into the well bore until the bit engages the bottom of the bore hole. The drill string is then rotated to rotate the bit and thus cut the hole. Drilling fluids are pumped through a swivel attached to the upper end of the square Kelly joint and down through the bore hole to cool the bit and carry the cuttings up through the annulus to the surface where the cuttings are removed from the drilling fluid before the fluid is recirculated. Since subsurface hydrocarbon fluid deposits are nearly always associated with super atmospheric pressure, and the drilling fluid is at atmospheric pressure when it is returned to the surface, the drilling fluid usually includes additives to greatly increase its specific gravity so that the column of liquid standing in the annulus results in a bottom hole pressure greater than the formation pressure to prevent blow outs. Since these weighted drilling fluids must be at a higher pressure than the formation pressure, the drilling mud migrates into the cracks and pores of the formation and adversely affects the porosity of these formations. Thus, after the bore hole is completed, the heavy drilling fluids must be swabbed from the bore hole and various chemicals and fracturing techniques must be used to again open the porosity of the bore hole and permit the hydrocarbon fluids to flow into the well bore and thus to the surface.
In more recent times, technologies have been developed to drill a well bore along a predetermined path so as to produce a slanted or even a horizontally extending bore hole. These methods generally include utilizing a bit driven by a hydraulic motor disposed in a bent housing so that the resulting bore hole traverses a slightly curving path. As a result of the motor driving the rotary bit relative to the drill string, the drill string does not have to be rotated to rotate the bit. After a predetermined increment the bore hole is cut, the drilling operation is interrupted, the mud swivel is removed, and a so-called steering tool lowered on a wireline by gravity and/or pumped into position by fluids until nested in a muleshoe or other means for establishing a predetermined position relative to the motor housing. The steering tool measures the degree and azimuth of inclination of the housing and the path of the bore hole can be plotted using a series of these measurements. The drill string can then be rotated from the surface to rotate the bent housing to a desired position so that the curving bore will return to or follow the desired path for the bore hole. The steering tool must then be removed from the drill string by the wireline, and mud circulation resumed to continue drilling the next segment of the bore hole.
Another technology has been developed for servicing wells under pressure so that the wells do not have be killed by pumping salt water or other heavy fluids into the well bore before undertaking the service operation. This technology is known as snubbing and involves a device for maintaining a seal around the tubing as it is mechanically forced into the well bore against the well pressure until such time as the weight of the workover string exceeds the force resulting from the well pressure multiplied by the cross sectional area of the workover string at which time the unit supports the tubing string as it is lowered into the well bore. Coiled tubing has been developed together with coiled tubing injectors for inserting the coiled tubing under pressure into the well bore. The coiled tubing is a single length of tubing, without joints, which is longer than the maximum depth of the bore hole to be penetrated. The coiled tubing may be inserted into and withdrawn from the well bore as a continuous operation which can be done at a much faster rate than the more conventional system utilizing individual joints of pipe. This is because the individual joints must be threadedly interconnected as the joints are successively injected or lowered into the well bore. This process is further slowed because the tool joints have a greater diameter than the pipe and must be successively passed through pressure locks to maintain the well pressure.
Various workover tools have been attached to the leading end of a downhole coiled tubing string, including various hydraulic motor driven rotating devices, and hydraulic fluids have been circulated through the tubing strings utilizing a swivel connection to the trailing end of the tubing string which is associated with the axle of the storage reel. Similarly, electrical cable which extends for the entire length of the coiled tubing has been used to electrically connect tools at the leading end of the coiled tubing string to surface instrumentation through an appropriate rotating electrical ring and brush contacts associated with the fluid swivel. Such workover operations are believed to have been exclusively performed within previously drilled bores, and normally within an existing pipe string such as the casing or production tubing.